1. Field of the Invention
This invention relates to a solvent recovery apparatus for recovery of solvents, such as thinner, which can conveniently employed in, for example, printing factories and painting works.
2. Description of the Prior Art
In printing factories and painting works, for example, large quantities of thinners, such as ink thinner and paint thinner, are used and, as a result, mixture gases containing such organic solvents are produced in high concentrations. From the standpoints of improvement of workshop environments, personnel health protection, in-plant explosion proofing, and environmental pollution control, these establishments require recovery of organic solvents from such mixture gases.
In such factories and works, exhausted gas in low concentration is produced by using paint. The low concentration exhaust gas is produced in large quantity, and in the case where organic solvents are recovered from such exhaust gas in a conventional device mentioned hereinafter, it is required that the device be large in size and the running cost is high.
For recovery of such organic solvents, it is known to employ apparatuses such as a fixed bed type recovery apparatus and a fluidized bed type recovery apparatus. A fixed bed type recovery apparatus includes a plurality of adsorption towers loaded with activated carbon which operate in two stages, an adsorption stage and a desorption stage, so that operation of individual towers can be changed over from the one stage to the other. A fluidized bed type recovery apparatus is of such an arrangement that a mixture gas stream is fed into an adsorption tower through a lower portion thereof and, in the course of its flow in the tower, solvent matter is recovered from the gas stream by being adsorbed on grains of an adsorbent, such as activated carbon, as the adsorbent flows downward in each of a plurality of adsorption stages provided in the tower along the vertical axis.
The foregoing prior art fixed-bed type solvent recovery apparatus requires a large installation space because it must have plurality of adsorption towers. Further, the flow rate of air in each tower should usually be relatively low, i.e., 0.2-0.3 m/sec, since an air flow rate in excess of such velocity range has the effect of lowered adsorption efficiency. Thus, since the rate of air flow is low, if solvents are recovered from the gases to be treated in low concentration, there occurs a problem that the adsorption towers must be excessively large in size, compared with the amount of recovered gases per unit time.
Another problem with such a solvent recovery apparatus is the pressure loss in the adsorption towers is considerably large, i.e., 500-800 mmAq, which fact requires an air blower and a pipe line, both of a large capacity, so that the apparatus is naturally of a large size, which in turn entails an increased cost of operation.
Another problem is that since grains of an adsorbent, such as activated carbon, on which solvents are adsorbed have to be transported over a comparatively long distance, in the case where a corrosive gas such as, for example, halogen gas is to be treated, anticorrosion treatment must be applied to the entire path of transport for the gas, which results in exceptionally high equipment cost.
In the case of the piror art fluidized bed type recovery apparatus, the rate of air flow is generally 0.6-0.8 m/sec, and pressure loss is 150-200 mmAq and, because of these facts, there have been encountered problems similar to those with the fixed bed type recovery apparatus.
With such prior art solvent recovery apparatuses, of both the fixed bed type and the fluidized bed type, as mentioned above, it has been the usual practice that mixture gases cointaining desorbed solvent removed from the adsorption and desorption towers a residual gas present after collected components have been condensed in a condenser, the exhausts of reactivating gases used for reactivation of adsorptive matter within the adsorption and desorption towers are released into the atmosphere after they are subjected to predetermined treatments, such as dust removal and cooling. Development of a technique for effective utilization of such exhaust gases has been demanded in the art.
In a known solvent recovery apparatus, of both the fixed bed type and the fluidized bed type, as mentioned above, the flow velocity of the gas stream is low, so that in the case where solvents are recovered from exhaust gases of a large capacity in low concentration, there occurs a problem that the arrangement is necessarily of a large size design. According to this third type prior art, gas that is to be treated is supplied at a certain position in the circumferential direction of the rotary adsorption device so that solvent contained in the gas is adsorbed on the rotary adsorption device and the adsorbed solvent is desorbed at a location on the downstream side in the direction of rotation of the rotary device relative to the position at which adsorption takes place.
In such a prior art solvent recovery apparatus, a rubber seal is used in order to provide airtightness between the rotary adsorption device and the housing in which the device is housed. That is, at least either the rotary adsorption device or the housing is provided with the rubber seal so that such airtightness is obtained by the rubber seal being airtightly held in contact with the housing or the rotary device.
With such prior air seal structure, however, the seal structure becomes readily damaged due to deformation or material deterioration with age of the rubber seal.
The rotary adsorption recovery body in such prior art, is so constructed that activated carbon in granular form fills the frame. It is known that the flow rate of the gas stream in the recovery apparatus using the rotary adsorption body is 0.2-0.3 m/sec, and the pressure loss is 300-600 mmAq, and therefore, problems similar to those with each type of solvent recovery apparatus in prior art as mentioned above have been encountered.
FIG. 1 is a flow diagram showing still another arrangement of solvent recovery apparatus. The solvent recovery apparatus if formed with a paper which is made from mixture of activated carbon in fiber form and cellulose, and possesses a rotary adsorption body 70 having a honeycomb construction. The honeycomb construction is in an appropriate state for air flow and the flow rate is increased in this prior arrangement.
In the case where the rotary adsorption body 70 in which solvent is adsorbed, is reactivated, cellulose has low durability and low burning resistances, so the solvent can not be desorbed directly with water vapor. Accordingly, it has been the usual practice that the solvent is desorbed from the rotary adsorption body 70 with hot air, and after exhausted gas is cooled by a cooling means 71, the solvent is adsorbed by an adsorption means 72. The adsorption means 72 is supplied with water vapor, and after desorbing solvent with water vapor, the exhaust gases are liquidized by condensation in a condensing means 73. The liquid obtained is a mixture of water and solvent and is separated by separating means 74, using the difference in specific gravity for example.
In such prior art, there is possibility that the flow rate is increased, but the rotary adsorption body 70 must be provided with the cooling means 71 and adsorbing means 72 on the outer portion of the body 70, which result in a large size of the arrangement and the cost of operation being high.